To the tourists, nothing's changed. They climb aboard the glass-bottom boats and chug across the bubbling bowl of water, peering down at fish and turtles swimming around as if they were in a giant aquarium.

Marwick, 68, moved to the Silver Springs area in 1970. An avid scuba diver, he explored the spring and the Silver River that flows from it. He found skeletons of mammoths and a prehistoric dugout canoe. He spearheaded the creation of the Silver River Museum and Environmental Education Center.

When he looks at Florida's most famous spring, he sees that the sparkling white sand on the bottom that gave the spring its namesake sheen has been smothered by globs of toxic algae. He sees mats of the brown glop floating on the surface, some of it so thick that alligators perch atop it. He sees a submerged tree that once glittered with shells — to the point it was dubbed "the Christmas tree" — is now a dull and fuzzy shade of olive.

"I used to look at the bank and see big patches of white sand and emergent plant life —- blues, reds, cardinal flowers," Marwick said. "It's all been smothered by the algae. The color palette that was the Silver River has just been washed out and now it's all brown and green."

Studies have found that 90 percent fewer fish are swimming in the spring compared to the 1950s. Pollution is increasing, the flow has dropped from 790 cubic feet per second (or about 510 million gallons per day) before 2000 to 535 cubic feet in the past decade.

"Silver Springs is headed for a slow, slimy, green death if we continue along this path," biologist Roy "Robin" Lewis said.

The cause of the water quality decline lies with the land around the spring. About 75 percent of the water gushing out of its 69 vents originates in runoff from a 4-mile radius. By analyzing aerial photos from the past 50 years, scientists in 2006 found that radius has changed from a predominantly natural landscape of forests and swamps into one that's homes, stores, roads and farms.

Runoff — full of fertilizer and septic waste — carried nitrate pollution into the spring, boosting the level twentyfold over the past century and fueling the thick algae growth.

But the loss of flow is what has captured the public's attention, especially after an 80-year-old Austria-born billionaire named Frank Stronach proposed pumping 25 million gallons of water a day from the aquifer for a 25,000-acre cattle ranch with its own slaughterhouse.

His Adena Springs Ranch permit has galvanized residents and environmental activists who fear Silver Springs is already being sucked dry by overpumping.

Stronach — who made a mint selling auto parts and is now backing a slate of candidates to take over the Austrian government — donated $1.5 million to the University of Florida to expand an agricultural research farm in Citra. This spring, he attended the ribbon-cutting for the Frank Stronach Plant Science Center. Also there were Senate President Mike Haridopolos, local county commissioners and the head of the Institute of Food and Agricultural Sciences.

Outside the gates, a crowd of protesters chanted, "No free water for cows!"

By then Stronach had already cut his request to 13 million gallons a day — still more water than the city of Ocala uses each day — and in August he reduced it to 5.3 million. Pump tests and computer models show that amount would lower the spring's flow by 0.03 percent, which would be "immeasurable," according to Stronach's attorney, Ed de la Parte Jr. of Tampa.

Meanwhile St. Johns River Water Management District experts have come up with a new theory about why the flow had dropped. They blamed an excess of vegetation that backed up water in the spring bowl — an explanation that had critics shaking their heads.

A bigger question, though, may be what's going to happen to all the waste that Stronach's 30,000 cows produce. According to de la Parte, all of that will be used to grow the grass that the cows will eat and "there will not be any excess" that could add to the pollution in the spring.

Water district officials aren't so sure, raising questions about possible sinkholes on the land. The deadline for Stronach responding to their concerns is Dec. 14.

This summer the state Department of Environmental Protection proposed limiting nitrates going into the spring to a quarter of what's in there now. Before a public hearing, DEP Director Herschel Vinyard Jr. and state Sen. Charlie Dean toured the spring and Silver River, but did not stick around to hear from the 150 people who wanted to comment on the proposal.

Vinyard also announced $1 million would be spent on such cleanup measures as moving a wastewater treatment facility that has been polluting the spring. However, Bob Knight of the Florida Springs Institute, who did his doctoral research on Silver Springs and the Silver River, scoffed that it's being moved "from one part of the springshed to the other. Oh yeah, that's gonna help."

Knight contends the state should spend hundreds of millions to clean up a spring that's so important to the local economy as well as to the drinking supply for the region. After all, he noted, Silver Springs has long been a Florida tourism icon.

"If you can't protect this one," he said, "what can you protect?"

Times researcher Caryn Baird contributed to this story, which includes information from the Gainesville Sun. Craig Pittman can be reached at craig@tampabay.com

CRAIG PITTMAN | Times

Glass-bottom boat tours are still a major attraction for visitors to Silver Springs, but what tourists see now is the springs’ decline. Pollution fueled algae blooms and driven out wildlife. There are 90 percent fewer fish compared to the 1950s.